The thermal dose is a measure of thermal tissue damage that is based on the Arrhenius equation and was originally introduced by Sapareto and Dewey in 1984 and is given byTD(t)=∫0τR43-T(τ)dτwhere
  R  =      {                                        0.25            ,                                                T            ≤            43                                                            0.5            ,                                                T            >            43                              and T is the temperature. The unit is typically given in equivalent minutes at 43 degrees Celsius. An increase of one degree (when above 43 degrees) doubles the thermal dose. This measure is currently the most prominently used measure for estimating when adequate thermal damage has been obtained in thermal therapies. A conventionally used limit for thermal necrosis in muscle tissue and uterine fibroids is 240 equivalent minutes at 43 degrees, although this limit has been found to be tissue dependent as some tissues are more sensitive to increases in temperature than other. Other measures for estimating the thermal damage also exist, such as the maximum temperature (which is very similar to the thermal dose for rapid heatings) and the Arrhenius equation as such. The thermal dose is typically applied in high-intensity focused ultrasound (HIFU) treatments whereas for example laser ablation often adopts the Arrhenius equation in its original form to assess thermal damage.
Whatever the measure, they have in common that they only use the measured temperature. The temperature can be measured by thermocouples, optical fibers, MR thermometry, US thermometry, thermoacoustic sensing, or any such means. These means of measuring the temperature have in common that they only measure the temperature at some points (thermo sensors), or in some planes (thermometry imaging, thermoacoustic sensing). The measurements may also be in 3D such as for 3D MR imaging but if so then the resolution is typically low and anisotropic to allow real-time acquisition.
For instance, magnetic resonance thermometry may be used to determine either the absolute temperature of a volume or a change in temperature, depending upon the technique used. For determining the absolute temperature several magnetic resonance peaks may be measured with spectroscopic imaging techniques. Methods which measure changes in temperature are typically faster and have been used to take temperature measurements for guiding thermal treatments. For example Proton resonance frequency shift based MR thermometry may be employed to provide temperature maps in water inside the tissue during the ablation procedure for real-time feedback control of the heating process.
In high-intensity focused ultrasound (HIFU) therapy, reliable real-time temperature monitoring using e.g. Magnetic Resonance Imaging (MRI) is necessary to ensure a sufficient thermal necrosis to the target while avoiding excessive heating and damage of surrounding healthy tissues. To achieve sufficient temporal and spatial resolution, fast imaging is required preferably with a high spatial resolution while maintaining a sufficient SNR for reconstruction of reliable temperature measurements.